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20th December 2009, 06:30 PM  #131  
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Cbe = gm * TF + Cje(Vbe) So there's really an implied gm term in the denominator of equation (11). Let's neglect Ccb for the moment, as it will be much less than Cbe. If the first term for Cbe above (= gm * TF) dominates as it does at high currents, then Cje(Vbe) can be neglected, and the gm terms in the numerator and denominator of the fT expression in equation (11) cancel (neglecting Ccb). In that case, fT is independent of current, having value 1/(2*pi*TF). So the slope really depends on the relative size of the two terms that make up Cbe above. If the second term dominates, fT will be nearly proportional to gm (and to Ic). I say "nearly proportional", because when Ic increases, Vbe increases also, which makes Cje(Vbe) increase somewhat. Of course, real transistors don't approach a constant fT at high currents. Rather fT falls off once it hits the peak. SPICE models this by making TF itself vary with current. Last edited by andy_c; 20th December 2009 at 06:59 PM. 

20th December 2009, 06:49 PM  #132 
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Andy 
is this just a coincidence? Analysis of the subtractive algorithm for greatest common divisors Regards,
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20th December 2009, 06:52 PM  #133 
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Here's one other thought. Surely, the fact that fT varies with current indicates nonlinear behavior, right? But who's the culprit here? Let's imagine that Cbe were completely constant, independent of Ic and Vbe. Surely that's a requirement for linearity too. But equation (11) shows that for a constant Cbe and Ccb, fT is proportional to current! Indeed, the way to make fT independent of current is to arrange for Cbe to be proportional to current just as gm is. So the culprit of this variation of fT with collector current is the fact that gm is proportional to collector current.

20th December 2009, 06:54 PM  #134  
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20th December 2009, 07:14 PM  #135  
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I was just comparing the devices that were mentioned here as being the same device, and where the japanese part was alleged to be superior. What are the more modern japanese transistors that you are referring to? I always like to do applesapples comparisons. BTW, I don't have the perception that Japanese data sheets are more complete or more accurate in their curves. Just look at the 2SC3264 LAPT datasheet; just one page. Crappy or not, I have not been able to get SPICE models for many Japanese transistors. My bottom line is that it is still usually dangerous to make generalizations, even though all of us are occasionally guilty of it. Don't get me wrong. The Japanese do great things very well. I drive a Lexus and will probably never buy an American car again, although that is probably unfair on my part. Cheers, Bob 

20th December 2009, 07:23 PM  #136  
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...I'am just finishing an amp using them. 

20th December 2009, 07:24 PM  #137 
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Sorry for making you confused, I thought if you were an author.
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20th December 2009, 07:25 PM  #138  
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BTW, in a power amplifier output stage, beta droop and ft droop at high current is a far bigger problem than at low current. Cheers, Bob 

20th December 2009, 07:32 PM  #139  
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Cheers, Bob 

20th December 2009, 07:37 PM  #140 
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Oops. That statement is an error. If Cbe were directly proportional to collector current as it is in the midcurrent region, and Ccb were negligible compared to Cbe, fT could be made constant with variations in collector current. This amounts to two nonlinearities canceling each other out. I probably should have said, "...independent of current in the lowcurrent region...".

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